Two of the six giant bridge building machines that will construct Halton’s new river crossing have been launched in the Mersey estuary, marking the latest milestone for the Mersey Gateway Project.

The form traveller machines have moved apart for the first time at the south pylon, enabling construction of the 1,000-metre long reinforced concrete bridge deck across the River Mersey to get underway.

The 270-tonne machines act as movable concrete moulds, operating in a similar way to the movable scaffold system, which is building the elevated approach viaducts.

The form travellers were assembled at the south pylon earlier this year before being lifted to their starting position around 25 metres above the riverbed. Construction teams then cast a pier table, a rectangular shaped platform, around the bridge pylon before preparing to start work on the main bridge deck.

Kyuyoung Choi, operations manager for the main crossing, explained the building process: “Each segment of the bridge deck is made in the same way. Reinforced steel is placed into the mould and we then pour around 130 cubic metres of concrete inside to create each segment.

“From the third segment onwards, we install the connection boxes, which are called ‘delta frames’, for the steel stay cables, which are then attached to the upper pylon. The form travellers, which are powered by a hydraulic system, then move forward on a set of rails to the next position and the process is repeated.

“The deck segments are cast simultaneously, which allows the bridge deck to ‘grow’ from either side of the pylons until it meets the connecting bridge deck and the structure is complete.”

Watch a video to see the form traveller machines in action:

Three pairs of form travellers are being used to build the bridge deck with each pair operating as a unit. The machines will be launched from the north pylon later this month and from the central pylon during October.

Work on the bridge deck is expected to finish in summer 2017. By this time the form travellers will have cast a total of 154 deck segments between them. Each segment of deck will be around 33 metres wide and six metres long and will take around a week to construct.

The largest number of deck segments will be cast from the south pylon. There will be 33 segments between the south pylon and the south elevated approach viaduct, and 31 cast from the south pylon towards the central pylon.

The north pylon will see 29 deck segments cast on one side and 27 on the other, while 17 will be cast from either side of the central pylon.

Hugh O’Connor, general manager of Merseylink, said: “This is going to be one of the most visually exciting parts of the project. As the structure of the main bridge begins to extend across the estuary it will completely transform the local landscape. Technically, it’s a very challenging process and this is where the international expertise from our highly skilled engineers will be invaluable, with many having worked on similar projects around the world.”

Cllr Rob Polhill, Leader of Halton Borough Council, said: “The three bridge pylons under construction are already giving us a glimpse of a much anticipated view of what will be become a landmark structure in our borough and indeed across the north west. The launch of the form traveller marks the next, thrilling stage of the project and I’m excited to see the bridge deck emerging across the river.”

The form travellers were designed and built specifically for the Mersey Gateway Project. When the bridge is complete, the machines will be dismantled and recycled.

The Merseylink Consortium was appointed by Halton Council as the project company in 2014, on a 30-year contract to design, build, finance and operate the project. Its equity partners are Macquarie Capital Group, BBGI, and FCC Construcción. The construction joint venture is made up of Kier Infrastructure & Overseas, Samsung C&T Corporation and FCC Construcción S.A. Sanef S.A. will deliver and operate the tolling solution for the consortium.